A review of catalytic hydrodeoxygenation of lignin derived phenols from biomass pyrolysis

A review of catalytic hydrodeoxygenation of lignin-derived phenols from biomass pyrolysis Quan Bua, Hanwu Leia,⇑, Alan H.. Lignins are highly substituted and mononuclear phenolic amorpho

A review of catalytic hydrodeoxygenation of lignin-derived phenols

from biomass pyrolysis

Quan Bua, Hanwu Leia,⇑, Alan H Zacherb, Lu Wanga, Shoujie Rena, Jing Lianga, Yi Weia, Yupeng Liua, Juming Tanga, Qin Zhanga, Roger Ruanc

a

Bioproducts, Sciences and Engineering Laboratory, Department of Biological Systems Engineering, Washington State University, Richland, WA 99354-1671, USA

b

Pacific Northwest National Laboratory, Richland, WA 99354-1671, USA

c

Department of Bioproducts and Biosystems Engineering, University of Minnesota, St Paul, MN 55108, USA

h i g h l i g h t s

"First review of the catalytic hydrodeoxygenation (HDO) of lignin-derived phenols

"Review of the applied HDO catalysts

"Review of reaction mechanism of HDO of lignin-derived phenols

"Review of the kinetics of HDO of different lignin-derived model compounds

"HDO-prospects and required further research and developments are discussed

a r t i c l e i n f o

Article history:

Received 10 July 2012

Received in revised form 21 August 2012

Accepted 22 August 2012

Available online 30 August 2012

Keywords:

Lignin-derived phenols

Catalytic hydrodeoxygenation (HDO)

Pyrolysis

Mechanism

Kinetics

a b s t r a c t

Catalytic hydrodeoxygenation (HDO) of lignin-derived phenols which are the lowest reactive chemical compounds in biomass pyrolysis oils has been reviewed The hydrodeoxygenation (HDO) catalysts have been discussed including traditional HDO catalysts such as CoMo/Al2O3and NiMo/Al2O3catalysts and transition metal catalysts (noble metals) The mechanism of HDO of lignin-derived phenols was analyzed

on the basis of different model compounds The kinetics of HDO of different lignin-derived model com-pounds has been investigated The diversity of bio-oils leads to the complexities of HDO kinetics The techno-economic analysis indicates that a series of major technical and economical efforts still have to

be investigated in details before scaling up the HDO of lignin-derived phenols in existed refinery infra-structure Examples of future investigation of HDO include significant challenges of improving catalysts and optimum operation conditions, further understanding of kinetics of complex bio-oils, and the avail-ability of sustainable and cost-effective hydrogen source

Ó 2012 Elsevier Ltd All rights reserved

1 Introduction

Progressive depletion of petroleum-based fuels has led to the

global energy crisis The greenhouse gas (GHG) emission

intro-duced by burning fossil fuels has made the global warming become

one of the biggest environmental challenges in human history (

Ni-gam and Singh, 2010) As one of the most significant renewable

re-sources on earth and the only renewable resource to be converted

to liquid fuel, biomass has recently received considerable

atten-tions when people search for alternatives of fossil fuel (Rocha

et al., 1996; Tang et al., 2009) The major components of biomass

are cellulose, hemicellulose and lignin Cellulose and hemicellulose

are consisted of complex polysaccharides (White et al., 2011)

Lignins are highly substituted and mononuclear phenolic amorphous polymers consisting of phenylpropane units, the precursors of which are three monolignols including p-coumaryl alcohol, conife-ryl alcohol, and sinapyl alcohol (Buranov and Mazza, 2008;

Cellulosic biomass can be converted into transportation fuels by three major pathways: syngas production by gasification, bio-oils production by pyrolysis or liquefaction, and aqueous sugar by hydrolysis (Huber et al., 2006) However, these are the initial steps

in the pathways towards transportation fuels; products upgrading/ separation is often more extensive than the primary conversions of biomass

Pyrolysis of biomass is a thermal decomposition process of or-ganic compounds in the absence of oxygen to obtain char, syngas, and bio-oil The bio-oil yield from fast pyrolysis is up to 60–70 wt.% which contains 15–30 wt.% of water (Brems et al., 2011; Velden

0960-8524/$ - see front matter Ó 2012 Elsevier Ltd All rights reserved.

⇑Corresponding author Tel.: +1 509 372 7628; fax: +1 509 372 7690.

E-mail address: hlei@tricity.wsu.edu (H Lei).

Contents lists available atSciVerse ScienceDirect

Bioresource Technology

j o u r n a l h o m e p a g e : w w w e l s e v i e r c o m / l o c a t e / b i o r t e c h

et al., 2008, 2010) Currently, bio-oil is considered as an alternative

to petroleum-based sources for a wide range of fuels and high

va-lue-added chemicals and other products (Yaman, 2004; Brems

et al., 2011) Bio-oil can be used as a liquid fuel or for the

produc-tion of chemicals and high grade transportaproduc-tion fuels after

upgrad-ing and separation Potential bio-based chemicals from bio-oil

include phenolics (Bu et al., 2012; Ren et al., 2012) and cyclic

ke-tones for resins and solvents, levoglucosan and levoglucosenon

for polymers, and aromatic hydrocarbons for fuels and solvents (

El-liot, 2004; Maher and Bressler, 2007)

With the development of lignocellulose-to-ethanol process and

lignin waste from pulp and paper refineries, recently a broad

inter-est to make full use of lignin for production of renewable fuels and

chemicals was developed Fast pyrolysis oil from lignocellulosic

materials is known to contain a complex mixture of phenolic

com-pounds which are the products primarily derived from the lignin

fractionation of the biomass Lignins are much more thermally

sta-ble than cellulose and hemicellulose during biomass pyrolysis

(Bridgwater, 1999) The compounds obtained from decomposition

of lignin have become one of the biggest challenges during

upgrad-ing of bio-oils Although there were a massive increasupgrad-ing interest in

biofuels production by bio-oil upgrading and the utilization of

lig-nin which is abundant from lignocellulose-to-ethanol process and

pulp and paper refineries (Adjaye and Bakhshi, 1995; Sharma and

Bakhshi, 1991; Chen et al., 2001; Mukkamala et al., 2012; Kleinert

and Barth, 2008; DeWild et al., 2009), there are just few major

reviews referring to the production of chemicals from lignin-derived

monomeric phenols via catalytic hydrotreatment (Pandey and Kim,

2011) This review focuses on HDO catalysts, HDO mechanism, and

kinetics study for catalytic HDO of lignin-derived phenols, which

will contribute to further study on production of biofuels and

chemicals by HDO of pyrolysis oils

2 Pyrolysis oil

2.1 Properties of pyrolysis oils

The pyrolysis oil also called bio-oils is a dark brown, free

flow-ing with smoky odor liquid from biomass pyrolysis (Massoth et al.,

2006; Lu et al., 2009) The physical properties of bio-oil show that

the water content, oxygen and ash contents of pyrolysis oil are

obviously higher than those in heavy petroleum fuel, and the

high-er heating value of pyrolysis oil is much lowhigh-er than that of

petro-leum fuel

The pyrolysis oils are complex mixtures of more than 300

com-pounds derived from the depolymerization and fragmentation

reactions of cellulose, hemicellulose, and lignin The major organic

compounds are acids, alcohol, ether, ketone, aldehyde, phenol,

es-ter, sugar, furan, and nitrogen compounds The phenolics

composi-tions (phenol, guaiacol and other substituted phenol composicomposi-tions)

are formed by decomposition of lignin, while the other oxygenates

such as sugar and furan should be formed from the

depolymeriza-tion of cellulose and hemicellulose of biomass The esters, acids,

alcohols, ketones, and aldehydes may be produced from

decompo-sition of the miscellaneous small oxygenated molecules (Huber

et al., 2006) As a result, the deleterious properties of high viscosity,

thermal instability, and corrosiveness present many obstacles to

the substitution of fossil derived fuels by bio oils (Zhang et al.,

2007; Diebold, 2000; Brems et al., 2011)

2.2 Instability of bio-oils

The instability of bio-oils is the result of their negative chemical

properties such as complex mixtures of chemical compounds and

high oxygen content (Brems et al., 2011) As a result, the viscosity

of bio-oils will be increased during storage and a fast increase will

be observed by heating Couples of complex chemical reactions may take place during aging of bio-oils (Diebold, 2000).Lu et al

compo-nents since they can react with lots of different chemical composi-tions, such as water, phenolics, alcohols, and protein Eventually, the properties of bio-oils will be changed with such kind of aging reactions, and these types of repolymerization reactions increased the average molecular of bio-oils, the viscosity, and the water con-tent (Lu et al., 2009; Mortensen et al., 2011).Oasmaa et al (2003)

reported that phase separation and gummy formation derived by pyrolytic lignin and char/coke formation at higher temperature were observed during aging reactions Therefore, these unfavorable characteristics of bio-oils make them difficult to be used Upgrad-ing is necessary before biomass pyrolysis oils can be used as com-petitive substitutes of petroleum fuels and chemical feedstocks in chemical industry

2.3 Lignin decomposition and conversion Lignin is the second most important components of lignocellu-losic biomass after cellulose, which is about 23–33% of the mass of softwoods and 16–25% of the mass of hardwoods (Mohan et al.,

2006) The lignin is composed of three different precursors: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol The guai-acol lignin which is derived from the polymerization of coniferyl phenylpropane unit is found predominately in softwoods materi-als The sinapyl unit is in hardwoods higher than that in softwoods The structure of lignin implicates that it can be considered as the sources of value-added chemicals such as phenolics (Pandey and Kim, 2011)

Lignin is the most heat-resistant component in the three major components of lignocellulosic biomass, and is typically decom-posed in the temperature range of 280–500 °C As a result, more residual char was produced during lignin pyrolysis than pyrolysis

of cellulose Phenols were yielded as the result of lignin pyrolysis through the cleavage of ether (mainlyaor b-o-4 bonds) and C–C linkages (Chen et al., 2001; Mohan et al., 2006)

The originality of lignin is very abundant with the development

of lignocelluloses-to-ethanol process, which makes use of cellulose and hemicellulose but leaves lignin as a waste (Pandey and Kim,

2011) Additionally, the conventional pulp and paper industries also generate a large amount of lignin Lignin is mainly being used

as a low-grade energy source in combustion (Huber et al., 2006) However, both the chemical structure and abundant originality

of lignin suggest that it has great potential to be used as value-added chemical feedstocks such as phenolic compounds, and the application of these products in chemical industry such as synthe-sis of phenol formaldehyde resin and plastics (Kleinert and Barth,

2008;Pandey and Kim, 2011) Furthermore, with the global energy crisis and the global warming issues, attentions have been increas-ingly paid to making liquid fuels and chemicals like aromatics and phenols by thermal treatment of lignin via depolymerization and conversion in hydrogen environment (Chen et al., 2001; Kleinert and Barth, 2008) Most studies on lignin decomposition and con-version have been simplified using lignin model compounds such

as monomeric, dimeric, and trimeric model compounds (Pandey and Kim, 2011;Chen et al., 2001)

3 Biomass pyrolysis oils upgrading

In order to overcome the deleterious properties of the biomass pyrolysis oil, the upgrading process is required before its applica-tion Currently, there are two major routes to make bio-oils being upgraded into a liquid transportation fuel: (1) catalytic

hydrodeoxygenation (HDO) (Furimsky, 2000), and (2) zeolite

upgrading The bio-oils upgrading is a complex reaction network

due to the complex compositions of pyrolysis oils (Adjaye and

Bakhshi, 1995; Mortensen et al., 2011) For these two routes,

cracking, hydrocracking, hydrodeoxygenation, decarboxylation,

decarbonylation, and hydrogenation have been reported to occur

(Mortensen et al., 2011) The degree of deoxygenation and cracking

of non-volatile components are the rate determining steps in

bio-oil upgrading (Adjaye and Bakhshi, 1995)

3.1 Catalytic hydrodeoxygenation (HDO)

HDO of bio-oil involves the presence of catalyst and hydrogen at

moderate temperature (300–600 °C) The oxygen is removed in the

form of water The overall reaction stoichiometry of HDO may be

schematically represented by (Mahfud et al., 2007):

AðCH2OÞA þ H2!AðCH2ÞA þ H2O

whereA(CH2O) represents the carbohydrate in the bio-oil

The reactions may take place during catalytic HDO process

including (Mortensen et al., 2011; Wildschut et al., 2009): (1)

water separation; (2) dehydration reaction due to the

condensa-tion polymerizacondensa-tion reaccondensa-tions; (3) decarboxylacondensa-tion reaccondensa-tion in

which oxygen is removed as the form of H2O; (4) hydrogenation

reaction which may involves the saturation of unsaturated

compo-nents; (5) hydrogenolysis reaction which is related to the breakup

of C-O bonds where the oxygen may be released in the form of

water; and (6) hydrocracking reaction involving the breakdown

of high molecular components into smaller molecules

3.1.1 Two-stage hydrotreatment process

two-stage hydrotreatment process This process was performed in a

down-flow (trickle bed) configuration consisting two fixed bed

reactors at 21 MPa The first reactor was operated below 300 °C

as the low-temperature stabilization reactor A subsequent main

hydrodeoxygenation occurred in the second reactor at higher

tem-perature (300–400 °C) The catalysts used were NiMo onc-alumina

and CoMo on spinel that were presulfided in the reactor before

feeding the bio-oil Furimsky (2000)pointed out that similar to

the upgrading of the coal-derived liquids (CDLs) which have more

than one stage required to obtain complete HDO because of the

high content of oxygenates and aromatics, a two stage operation

is beneficial for conversion biomass pyrolysis oil to quality

com-petitive transportation fuels The first stage is performed at

tem-perature below 300 °C with sulfided CoMo or NiMo catalyst to

remove O-compound and produce a stabilized oil product (Elliott,

2007; Furimsky, 2000) The second stage operated at the

condi-tions of about 350 °C and 13.8 MPa In the second stage, furans

and phenols are the predominant oxygen compounds; because of

the low HDO reactivity of these compounds, a higher reaction

tem-perature and higher H2pressure are required to convert them into

oxygen free products (Furimsky, 2000) However, the catalyst life

of this process is poor due to breakdown of supports

3.2 HDO catalysts

Table 1shows the overview of catalysts for HDO of different

model compound feeds Industrial hydrodesulfication (HDS)

cata-lysts (e.g Co–Mo or Ni–Mo Al2O3) which are used for removal of

sulfur, nitrogen, and oxygen from petroleum fuels are utilized as

HDO catalysts (Huber et al., 2006; Zhang et al., 2007; Mortensen

Co-MoS2and Ni-MoS2as catalysts (Furimsky, 2000; Weigold, 1982;

Bui et al., 2011a,b; Massoth et al., 2006; Wildschut et al., 2009)

For these traditional HDS catalysts, Co and Ni serve as promoters

which donate electrons to the active sites – Molybdenum Morte-sen et al proposed that this contributed to weaken the bond be-tween molybdenum and sulfur so that a sulfur vacancy site is generated, and these sites are the active sites for both HDS and HDO However, unsulphided form of the catalyst showed less activity than the sulphided catalysts (Wildschut et al., 2009), which indicates that the addition of a sulfur source will be benefi-cial for these catalysts

Recently, some transition metal based catalysts including noble metals such as Pt, Pd and Ru and carbon based catalysts are utilized

by some researches (Wildschut et al., 2009; Venderbosch et al., 2010; Zhao et al., 2011; Gutierrez et al., 2009; Centeno et al.,

1999) Wildschut et al investigated the effects of the noble-metal catalysts Ru/C, Pt/C, Pd/C on HDO of bio-oils in a batch reactor un-der a two stage operating conditions, mild HDO at 250 °C and

100 bar and deep HDO at 350 °C and 200 bar The results showed that both the yields and the degree of deoxygenation were higher for the noble-metal catalysts than those for the traditional CoMo/

Al2O3 and NiMo/Al2O3 catalysts According to the combinations

of oil yields, levels of deoxygenation and the extent of hydrogen consumption, they proposed that Ru/C might be the most promis-ing catalyst for further study of catalytic HDO.Mortensen et al (2011)pointed out that the transition metal catalysts should be bifunctional based on the mechanistic speculations However, whether the activation of oxy-compound is achieved through the metal sites or at the metal support interface is still unclear though generally agreed that the metals contain the hydrogenation active sites (Mortensen et al., 2011) There are still some problems rele-vant to deactivation of catalysts led by coking and metal decompo-sition (Furimsky and Massoth, 1999) During catalytic HDO using traditional HDS catalysts, Al2O3shown to be unsuitable support because the water formed in HDO and the oxygen containing com-pounds may convert it to boehmite (AlO(OH)) (Mortensen et al.,

showed that an indication of high affinity for carbon formation ex-isted on the alumina support because of the relatively high acidity

of Al2O3(Popov et al., 2010) As a result, carbon has been found to

be a promising alternative to Al2O3due to its neutral nature and high surface area (Mortensen et al., 2011; Wildschut et al., 2009) 3.3 Deactivation of HDO catalysts

Furimsky and Massoth (1999)pointed out that there are three stages for catalyst deactivation: initial deactivation caused by coke; metal deposition; and pore constriction and ultimate pore blockage It is known that coking is one of the major challenges for HDO reaction; and coking has close relationship with the struc-ture of catalyst (Netzel et al., 1996) since polymerization reactions which are the major reactions leading to coking occur on the cata-lyst surface, which significantly reduces the active sites by compe-tition with the reactant As a result, the properties of the adsorbed feed have an important effect on coke formation For the feeds with high content of coke precursors, Laurent et al (1994) demon-strated that some phenols are important coke precursors, such as methoxy-phenol, hydroxyphenol, phenol or methylanisol, the pre-treatment of the catalyst is necessary (Furimsky and Massoth,

1999)

Furimsky and Massoth illustrated that besides feed composition and catalyst properties, the severity of conditions is another impor-tant factor that affects the extent of coke formation, such as hydro-gen pressure, reaction temperature and contact time.Gualda and Kasztelan (1996)found that the coking extent decreased with the increase of hydrogen pressure, reaction temperature and contact time, which was the result of removal of coke precursors and an improved hydrogenation of the coke that deposited on the catalyst surface Therefore, the design of active and stable HDO catalysts

that can be used with minimum hydrogen consumption and mild

reaction condition is still a big challenge

4 Mechanism of HDO of phenols

Due to the complex compositions of bio crude oils, most of the

HDO studies reported have been focused on using model

com-pounds rather than the real bio crude oils Phenols have been

re-ceived considerable attentions because of their low reactivity in

HDO process Therefore, the HDO of phenols becomes one of the

key reactions in pyrolysis oil upgrading A large portion of the work

have been carried out to avoid the rings becoming saturated so as

to minimizing the hydrogen consumption in HDO of model

com-pound using phenols (Weigold, 1982; Bui et al., 2011a,b; Ahmad

et al., 2010; Sepulveda et al., 2011) Apart from minimizing the

hydrogen consumption by retaining the aromatic character of the

upgraded oils, the unsaturated aromatics have a much higher

oc-tane value comparing with the saturated cyclic hydrocarbons

(Elliott, 2007).Senol (2009)reported that two reaction routes were

observed during HDO of phenol: (a) the aromatic ring has been

re-tained and the CAromAO bond is hydrogenolysed; and (b) the

aro-matic ring is hydrogenated to be saturated before cleavage of the

carbon-oxygen bond.Baker and Elliott (1993)) developed a

multi-step process in which the initial hydrotreating for oxygen

reduc-tion process was followed by separareduc-tion of different components;

and a second operation was designed for hydrocracking the heavy

components The two-step operation was possible to maximize the

aromatics by removing the aromatic fraction before it became

sat-urated; moreover, the hydrogen consumption was minimized by

retaining the aromatic character

Most phenols can be divided into two types in pyrolysis oils: the

substituted phenols with alkyl such as cresol, and methoxyl such

as guaiacol Therefore, most of the attentions have been paid to

study the HDO of alkyl substituted phenols and HDO of guaiacol

as the model compounds (Odebunmi and Ollis, 1983; Bui et al.,

2011a,b)

4.1 HDO of alkyl phenols

using sulphided CoMo/Al2O3catalysts at low temperatures ranging

from 220 to 270 °C and high temperatures in the range of 350–

400 °C The order of HDO reactivity for substituted phenols was:

meta > para > ortho; this should have close relationship with the

steric effect of o-substitution of phenols (Furimsky, 2000) Results

revealed that toluene and cyclohexane were the main products

Subsequently, Wandas et al (1996)investigated the conversion

of cresols in the presence of sulfide CoMo catalyst in a stirred

auto-clave at the temperature of 360 °C and hydrogen pressure of 7 MPa

They observed that the highest conversion was obtained for

p-cresol, the o-cresol was least effective, methyl cyclohexane was the main product, toluene was the second main product, and the toluene yield for m-cresol and o-cresol was 27 and 11 mol.%, respectively The main products distribution and total conversion

of cresols were shown in Fig 1 These results were in good agreement with results from Odebunmi and Ollis (1983), which also implied that the reaction of ring saturation was prior to the oxygen removal in the presence of CoMo/Al2O3catalyst Otherwise, toluene would be the main product as the direct deoxygenation (DDO) reaction was predominant

In general, there are two parallel pathways for HDO of alkyl substituted phenols: direct deoxygenation (DDO) with the aromatic character retained and hydrogenation (HYD) with the aromatic ring being hydrogenated to be saturated.Massoth et al (2006)carried out the catalytic HDO of methyl-substituted phenols

in a flow microreactor at 300 °C and 2.85 MPa hydrogen pressure

in the presence of sulfide CoMo/Al2O3 The observed primary reac-tion products include methyl-substituted benzene, cyclohexene, cyclohexane and H2O Their results analysis suggested two paths for HDO of phenols, the aromatics path and partially or completely cyclohexanes path, which is consistent with Odebunmi and Ollis’ conclusions (Odebunmi and Ollis, 1983)

Yang et al (2009)investigated HDO of phenol in supercritical hexane at different temperature range (300-450 °C) and 5.0 MPa

of hydrogen pressure with sulfide CoMo and CoMoP catalysts sup-ported on MgO The results indicated that the CoMoP/MgO catalyst showed superior activating in HDO of phenol; the highest yield of benzene was obtained at 450 °C and reaction time of 60 min using CoMoP/MgO catalyst The obtained products yields distribution re-veals that the HDO activity of this catalyst increased drastically with the increase of the reaction temperature

Table 1

Overview of catalysts used for HDO of model compound.

Feed Set-up Catalyst Support T (°C) P (MPa) References

o-, m- ,p-Cresols Batch CoMo/Al 2 O 3 Al 2 O 3 360 7 Wandas et al., 1996

Phenol Batch NiMo/Al 2 O 3 Al 2 O 3 350 7.5 Ryymin et al., 2010

4-Methyl-acetophenol, ethyldecanoate, guaiacol Batch CoMo/C C 280 7 Puente et al., 1999

Guaiacol Batch Pt/ZrO 2 , RhPt/ZrO 2 ZrO 2 350 1 Gutierrez et al., 2008

Phenol Continuous NiW/C C 250–300 1.5 Echeandia et al., 2010

Pyrolysis oil Batch Noble metal C, TiO 2 250–350 10–20 Wildschut et al., 2009

Pyrolysis oil Batch Ru/C C 350 20 Yoosuk et al., 2012

Phenol Batch Amorphous NiMoS 350 2.8 Odebunmi and Ollis, 1983

Cresol Batch CoMo/Al 2 O 3 Al 2 O 3 375–400 6.9 Yang et al., 2009

Methyl-phenols Continuous CoMo/Al 2 O 3 Al 2 O 3 300 2.85 Massoth et al., 2006

Phenol Batch CoMo/MgO, CoMoP/MgO MgO 300–450 5 Ahmad et al., 2010

Guaiacol, cresols Continuous Pt/ZSM-5/, Pt/Al 2 O 3 ZSM-5, Al 2 O 3 200 4 Wang et al., 2012

Fig 1 Total conversion and main products yield distribution of cresol as model compound (360 °C, 7 MPa, 60 min) The figure is drawn based on data from Wandas

Weigold (1982)carried out the direct HDO (CAO bond scission

but the aromatic character retained) of ring alkyl-substituted

phe-nols at temperature of 400 °C and 7.6 MPa hydrogen pressure in

the presence of a commercial CoMo/Al2O3 The results indicated

that the catalytically active site is stereochemically demanding in

the HDO of phenols He pointed out that the DDO and HYD reaction

proceed on the same catalytic site and the DDO path was retarded

mainly if the transfer of the substrate hydroxyl group onto a

coord-inatively unsaturated metal site of the catalyst has been inhibited

4.2 HDO of methoxyl-substituted phenols

Other significant substituted phenols of pyrolysis oil are

meth-oxyl-substituted phenols such as guaiacol (GUA) Studies on

cata-lytic HDO of guaiacol as model compound of bio-crude have

received increasing attention in the past Bui et al compared the

effects of unsupported and alumina supported MoS2and CoMoS

catalysts on HDO of GUA at temperature of 300 °C and 4 MPa

hydrogen pressure (Bui et al., 2011a) The results verified that

the DDO pathway for GUA conversion was obviously increased

with the CoMoS catalyst compared to the non-promoted MoS2

cat-alyst and this effect is similar to hydrodesulfurization (HDS)

pro-cess In a continuous study, Bui et al studied the support effect

for CoMoS catalysts in HDO of GUA (Bui et al., 2011b) Three

differ-ent supports, Al2O3, TiO2, ZrO2, were tested to investigate the

sup-port effect of CoMoS catalysts on the selectivity and HDO activity

of GUA The effects of supports on selectivity in respect to GUA

conversion revealed that there were slightly different for the

pri-mary products of transformation depending on different supports,

for example, for CoMoS/Al2O3, catechol (CAT) and phenol were

formed at low conversion rates; only CAT formed at low conversion

rates on CoMoS/TiO2; however, phenol was the main product for

all different GUA conversion on CoMoS/ZrO2 Besides, it was

ob-served that the benzene yield was much higher on ZrO2supported

catalyst compared with other two different supported catalysts

There are two parallel reaction pathways for GUA conversion:

one is that phenol can be obtained directly with the effects of

demethoxylation (DMO) of GUA; another one is demethylation

(DME) in which catechol is produced as an intermediate, and then

under the performance of methyl substitution and dehydration,

phenol and alkyl substituted phenols are generated (Bui et al.,

2011a,b).Li et al (2011)pointed out that the similar mechanism

for GUA conversion might be applicable when they investigated

the influence of phenols on bio-oil upgrading over Pt/Al2(SiO3)3,

Pt/C or Pt/MgO catalyst

Ahmad et al (2010)investigated the possible routes and

devel-oped the feasible process route for upgrading of bio-oil into

value-added transportation fuel via HDO process by simulation in

PETRONAS iCON software They proposed a three-stage process for

upgrading of bio-oil via HDO by using GUA as model compound

The first stage consists of the conversion of GUA to catechol

(CAT) by hydrogenolysis; the second stage involves hydrotreating

of CAT to phenol; and the third stage is also called the stabilization

stage by which value-added hydrocarbon such as benzene,

cyclo-hexane and cyclohexene formed from the HDO of phenol The

re-sults indicated that the selection of model compounds was a

significant challenge for bio-oil upgrading and this simulation

implicated the feasibility of HDO using model compounds to

pro-duce value-added transportation fuels However, there are some

disadvantages related to model limitations such as the complexity

of the simulation due to the selection of feed phases and operation

parameters and the designated unit operation

Gutierrez et al (2009)discussed the HDO of GUA as pyrolysis oil

model compound using zirconia-supported noble metal catalysts

They observed that temperatures higher than 300 °C are required

for HDO of GUA, which was in good agreement with the results

reported byYang et al (2009) The noble metal catalysts have potential to be a promising alternative to traditional sulfide cata-lysts due to the low carbon deposition on the noble metal catacata-lysts However, the low cost HDO catalysts should be the target of any further studies on HDO of bio-oils

5 Kinetics of HDO of phenols Bio-oil upgrading involves a complex combinations of parallel and series reactions in which the reaction constants and other parameters vary according to the operating conditions, for exam-ple, apparent activation energy values indicate that bio-oil is par-ticular sensitive to reaction temperature (Adjaye and Bakhshi,

1995) The kinetics of HDO has been investigated to better under-stand the HDO mechanism of pyrolysis oil However, sparse reli-able kinetic information of HDO of pyrolysis oil is reported due

to the diversity of complex compositions of bio-oils, instead a vari-ety of studies explored the HDO kinetics of different model com-pounds which were found to be the significant compositions of bio-oils with lower reactivity, such as phenols, furans, aldehydes (Massoth et al., 2006; Zhao et al., 2011; Odebunmi and Ollis, 1983; Sitthisa et al., 2011; Prochazkova et al., 2007) The experi-mental conditions used, such as catalyst, types of reactors, method

of analysis, will have influence on the kinetic parameters obtained

by different studies (Furimsky, 2000)

The kinetics of HDO of phenol and substituted phenols (used as model compound of bio-oils) have received increasing attentions

in the past years since the existence of phenols in pyrolysis oil sig-nificantly affect the results of hydroprocessing of pyrolysis oil be-cause of their low reactivity and the abundant originalities of lignin-derived phenols Furimsky pointed out that most kinetics studies of HDO of phenols were performed in trickle bed reactors and batch reactors The commercial sulphided CoMo and NiMo cat-alysts were favorable used catcat-alysts (Furimsky, 2000)

Odebunmi and Ollis (1983)carried out a study on catalytic HDO

of m-cresol in a packed bed trickle reactor over CoMo HDS cata-lysts A Langmuir-Hinshelwood kinetic model is developed to illus-trate the data:

Fc

dXc

dW

¼ kHDOKHKcCcPH ð1 þ KcCcþ KWCWþ KSCSÞ (HDO of m-cresol in a cresol-only feed) (S = mercaptan, W = water,

c = cresol and H = hydrogen)

On the basis of the assumptions (KWCWis neglected since water inhibition was found not to be significant; KHPH(constant) is incor-porated into kHDO), the alternative expression of this equation is: W

Fco

¼  1 þ kcCco

kHDOKcCco

lnð1  XcÞ

¼  1

k0HDOCco lnð1  XcÞ The HDO of m-cresol shows apparent first-order dependence on m-cresol concentration both in m-cresol-only and the mixed feed

as shown inTable 2.Li et al (1985)addressed the hydrodeoxygen-ation of phenolic compounds in the acid fractions over a sulfided

Table 2 The rate equation parameters for CRE ( Odebunmi and Ollis, 1983 ).

Temperature (°C) lnk HDO K c (CRE) (L/mol)

Ea (kcal/mol) 20.2 DH c = kcal/mol

lnA 9.0 DH c = 11 kcal/mol

Ni–Mo/Al2O3catalyst at 350 °C and 120 atm The first-order

kinet-ics for the overall conversion of phenolic compounds shows well

for the HDO process The pseudo-first-order rate constants indicate

that the rate constants for each of the compound are almost the

same (about 104L/(g of cat s)) The authors claimed that the

HYD of phenolic compounds is faster than direct HDO under partial

conditions though the data were not sufficient to demonstrate the

competitive inhibition effects in these hydroprocessing reactions

competi-tors on activity and selectivity of HDO of phenols over sulfide

Co–Mo/Al2O3 and Ni–Mo/Al2O3 catalysts in a batch reactor at

350 °C and 7 MPa of hydrogen pressure The results showed that

the conversion of model compound (4-methylphenol (4MP))

fol-lows apparent-first-order kinetics in this process The

pseudo-first-order rate constants used byGevert et al (1987)are used in

this study:

 ln Ci

C0

 

¼ k4MPWf t

V

 

f t

V

 

¼Xn

i¼1

ti ti1

Vi1

where Ci and C0 are the concentrations of 4-methylphenol in

samples i and 0; k4MP is the apparent-first-order rate constant

(cm3min1g1); W is the catalyst weight (g); n is the number of

samples taken; t is the time (min); and V is the solution volume

(cm3)

The linear regression for the determination of the rate constants

has demonstrated the validity of this equation A

Langmuir–Hin-shelwood equation is used as kinetic expression of the inhibition:

k0app

kapp

¼ 1 þ KiC0

where k is the apparent-first-order rate constant without

competi-tor in the feed, and kappis the apparent-first-order rate constant at

an initial competitor concentration CO Kiis the inhibition constant

presumed to represent the adsorption of the competitor on active

sites

The results demonstrate that there are two parallel reaction

pathways for HDO of phenols: the direct hydrogenolysis of the

Csp 3AO bond (DDO), and the hydrogenation of the aromatic ring

(HYD), which is in good agreement with literature reports (Bui

et al., 2011a,b; Senol, 2009) The observations show that, in all

cases, the hydrogenolysis path (DDO) is more inhibited than the

hydrogenation path (HYD), which is consistent well with Li

et al.’s (1985)conclusion that HYD of phenolic compounds is faster

than direct HDO under partial conditions

Massoth et al (2006) discussed the correlations of kinetics

parameters with molecular properties during catalytic HDO of

methyl-substituted phenols The Langmuir–Hinshelwood kinetics

was used to analyze the obtained reaction data Their observation

also verified the presence of the two reaction pathway-HYD and

CAOH (DDO) It is observed that the adsorption constant was

found to be the same for both reaction paths, indicating that a

sin-gle catalytic site center is operative for both reactions They

pro-posed that it was the substitutes’ effects on the electrostatic

potential and orbital rather than the steric effects that resulted in

the dependencies of adsorption and reaction rates upon methyl

substitution

6 Overall aspects and prospect of catalytic HDO of

lignin-derived phenols

Although a huge number of researches focused on catalytic HDO

of lignin-derived phenolic compounds from biomass pyrolysis in

lab’s perspective, further study to explore integration of existing oil refinery infrastructure for HDO of lignin-derived phenols in an industrial prospective is necessary It is known that hydrogen con-sumption is one of the most significant factors that affect the cost

of final products during HDO of bio-oils

Currently, 96% of hydrogen used is produced from fossil fuels (Geantet and Guilhaume, 2008) However, considerable attentions have been paid to hydrogen production from biomass in the past years (Spath et al., 2005; Milne et al., 2002; Ni et al., 2006); for example, National Renewable Energy Laboratory (NREL) had devel-oped two Aspen Plus models of hydrogen production via gasifica-tion; their preliminary screening study showed the potential of hydrogen to be an economic feasible product from biomass-de-rived syngas In general, hydrogen can be produced from biomass

by different routes, such as gasification, hydrogen from biomass-derived methanol and ethanol via steam reforming (Milne et al., 2002; Geantet and Guilhaume, 2008; Spath et al., 2005), and cata-lytic steam reforming of pyrolysis oil (Geantet and Guilhaume,

2008;Spath et al., 2005) Pyrolyis oil can be separated into two fractions according to water solubility; the oxygenated fraction

or water-soluble fraction can be used for hydrogen production after water-insoluble fraction have been extracted for adhesive for-mulation (Ni et al., 2006; Wang et al., 1997) Marquevich et al

reforming of model compounds of pyrolysis oil with commercial nickel-based catalysts They pointed out that the temperature played a significant role in the process; for example, the model compounds (acetic acid, m-cresol, and dibenzyl ether) were almost completely converted to hydrogen and carbon oxides with temper-atures above 650 °C However, carbonaceous deposits on catalyst for acetic acid were observed when temperatures were lower than

650 °C Results suggest that it is difficult to reform sugars due to their rapid decomposition during the process and char forming in the reactor freeboard

reactions for catalytic production of hydrogen from bio-oils, including:

Steam reforming CxHyOzþ ðx  zÞH2O ! xCO þ ðx  z þ yÞH2; partial oxidation CxHyOzþ ðx  zÞO2! xCO þ ðy

2ÞH2; and catalytic cracking For catalysts used for hydrogen from biomass pyrolysis oil, Ni-based catalysts were favored in hydrogen production from biomass-derived pyrolysis oil (Milne et al., 2002; Ni et al., 2006; Wang et al., 1997) Milne et al proposed the overall stoichiometry for catalytic steam reforming of bio-oil at 750–850 °C over a nickel-based catalyst, which gives a maximum yield of 17.2 g H/100 g bio-oil (Milne et al., 2002)

Wright et al (2010)had developed two 2000 MT/day corn sto-ver scenarios for bio-oil upgrading to transportation fuels: one is hydrogen generated from aqueous phase of bio-oil for fuel upgrad-ing; another one is hydrogen purchased from an external source Results show that the products value (PV) for hydrogen production scenario is $3.09/gal of fuel which includes a bio-oil production cost of $0.83/gal Fuel yield for this scenario is 134 million liters

of fuels per year However, the PV is $2.11/gal of fuels for the hydrogen purchase scenario of which the purchase price of hydro-gen considered is $1.5/kg The fuel yield for this scenario is 220 million liters of fuels per year The capital expenditures for hydro-gen production and merchant hydrohydro-gen are estimated at $287 mil-lion and $200 milmil-lion, respectively, indicating that the difference of

PV between these two scenarios is derived by the high capital costs for hydrogen production due to additional equipment and installa-tion costs Besides, the low fuel yield which resulted from less bio-oil left for upgrading during hydrogen production scenario is another most significant factor that influences the PV Sensitivity analysis suggested that an increase of fuel yields by slightly improved bio-oil upgrading processing will reduce the fuel cost

significantly; for example, a variation of 5% in the bio-oil upgrading

will result in PV of $2.60–3.89 for hydrogen production scenario

(Wright et al., 2010) Undoubtedly, other factors, such as feedstock

properties which may affect bio-oil quality and catalyst

perfor-mance, will also impact the product yields and require further

study

UOP (Marker et al., 2005) hydrotreated dynamotive pyrolytic

lignin and aimed to produce aromatics and gasoline The obtained

results suggest that pyrolytic lignin can be hydrotreated under

mild conditions and further process and more active catalyst

should be developed The adiabatic pre-reforming technology

was employed to integrate reforming of aqueous phase of bio-oil

into an existing hydrogen plant for hydrogen production for

upgrading of pyrolysis lignin phase However, because of aqueous

phase can account a large amount (70%) of bio-oil weight, which

only leaves a small amount (30%) of bio-oils for upgrading and

leads to the low yield of fuels The original UOP analysis was

mod-ified byWright et al.’s (2010)assumption that only a smaller

frac-tion (38%) of bio-oil is needed to be reformed for providing

hydrogen to upgrade the remaining bio-oil

Marker et al (2005) indicated that producing gasoline from

pyrolytic lignin will become economically attractive at lower crude

oil prices when pyrolysis oil price decreases Furthermore,

although the current fuel cost is lower for hydrogen purchase

sce-nario, it is quite possible that the hydrogen selling price will have

significant effect on fuel cost Therefore, it still has great potential

to explore cost effective hydrogen production scenario for bio-oil

upgrading process A summary of needs to achieve the goal for

pro-ducing fuels from catalytic HDO of pyrolytic lignin-derived phase

and general upgrading of bio-oil is as follows: (1) reduce capital

cost for hydrogen production scenario by making use of the

exist-ing refinery units; (2) develop more active catalyst with good

cat-alyst lifetime and investigate the limitations of reactors for scale up

of bio-oil upgrading using existing refining units; (3) increase the

yield of fuel by improvement of upgrading processing; and (4)

fur-ther research is required to lower the high acid number of bio-oil

which may lead to serious corrosion in standard refinery units

7 Conclusions

Catalytic hydrodeoxygenation (HDO) of lignin-derived phenols

which are the lowest reactive chemical compounds in pyrolysis

oils has been reviewed Catalysts having both high capabilities to

avoid carbon formation and a sufficient activity for HDO are of a

great challenge The techno-economic analysis of catalytic HDO

and bio-oil upgrading implies that further study on major technical

and economic efforts are required to develop transportation fuel

route from bio-oils, which include: (1) investigating improved

cat-alysts; (2) exploring alternative hydrogen source; (3) detailed

kinetics study; (4) optimizing the reaction conditions to make

HDO of biomass derived phenols in the existing refinery

infrastruc-ture possible

Acknowledgements

This work was supported in partial by the Office of Research

and Department of Biological Systems Engineering at Washington

State University

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